The PET polyesters are excellent in mechanical strength, heat resistance, transparency and gas barrier properties, light in weight and inexpensive, so that they are widely used for bottles, sheets, films, fibers and foam articles. In particular, they are suitable as filling vessels for drinks and foodstuffs such as carbonated drinks, fruit juice drinks, liquid seasonings, cooking fat and oil, sake and wine.
Such PET polyesters are each produced by using a terephthalic acid component and an ethylene glycol component as main components, esterifying the terephthalic acid component with the ethylene glycol component, and then, conducting melt polycondensation usually in the presence of a polycondensation catalyst.
The PET polyester. thus produced contains a considerable amount of CTET, so that usually, the molecular weight is increased and the CTET is simultaneously reduced by solid-phase polymerization. However, it is known that, even if the CTET is reduced by solid-phase polymerization, the CTET is reproduced by melt extrusion in forming processing to cause an obstacle in a manufacturing process of a product of said polyester. For example, when hollow vessels such as bottles are produced, the CTET is reproduced in injection molding, which raises the problem that a die is contaminated with the CTET, or that the PET polyester is contaminated with the CTET as foreign matter to deteriorate qualities of the vessels. In the case of melt spinning, the CTET accumulates around an exit of a nozzle to cause bending of the extruded polymer at the exit, yarn breakage or yarn unevenness. Further, when films are produced, the CTET is reproduced in melting the polyester, which introduces the problem of film breakage or deterioration of qualities.
Accordingly, in order to solve the above-mentioned problems with regard to the CTET, it is necessary not only to decrease the CTET content by solid-phase polymerization, but also to inhibit formation of the CTET in melting to prevent an increase in the CTET content.
For solving these problems, various methods have hitherto been attempted. For example, JP-A-6-322082 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a method of copolymerizing a monofunctional component having a hydroxyl group or a carboxyl group with a PET polyester. In this method, however, a terminal functional group is blocked with the monofunctional component in melt polycondensation, so that the polycondensation reaction is inhibited to cause failure to obtain a PET polyester having a high intrinsic viscosity.
Further, methods for producing PET polyesters through melt polycondensation and solid-phase polymerization processes, in which the polyesters are treated with hot water or water vapor after the solid-phase polymerization process to inactivate polycondensation catalysts contained in said polyesters, thereby inhibiting formation of the CTET in melt forming are proposed (U.S. Pat. Nos. 5,241,046 and 5,444,144, JP-A-5-97990 and JP-A-6-184286). However, these methods have the problem that hot water treating apparatuses or water vapor treating apparatuses are required in addition to polymerization apparatuses, for inactivating the polycondensation catalysts, resulting in high cost.
The present invention provide a PET polyester composition which is decreased in the amount of the CTET formed in melting, and a method for producing the same.